This invention relates to the investigation of the properties of earth formations surrounding a borehole and, more particularly, to an apparatus and method for determining the dielectric constant and/or conductivity of formations surrounding a borehole using radio frequency electromagnetic energy. The subject matter of the present invention is related to subject matter disclosed in the copending U.S. Patent Application Ser. No. 835,127, of G. Huchital and J. Tabanou, filed of even date herewith and assigned to the same assignee as the present invention.
It is well known to log or record certain electrical characteristics of earth formations surrounding a well borehole as a function of depth in order to determine the location and extent of oil-bearing strata. A log of formation resistivity versus depth may indicate the presence of hydrocarbons, since hydrocarbon-bearing formations typically exhibit a higher resistivity than formations containing mostly salt water. If the formation connate water is relatively fresh, however, there can be ambiguities in interpreting results since there may be insufficient contrast between the resistivity of the hydrocarbons and the resistivity of the water.
Ambiguities of resistivity logs in fresh water zones and other factors have led to an increasing interest in the development of techniques for obtaining measurements of the dielectric constant or electric permittivity of subsurface formations. The dielectric constant of different materials commonly found in earth formations vary widely. For example, the dielectric constant of oil is on the order of 2.2 while the dielectric constant of limestone is on the order of 7.5. In contrast, the dielectric constant of water is on the order of 80 and is largely independent of the salinity (and resistivity) of the water. Thus, measurement of dielectric properties of formations holds much promise of being a useful means of formation evaluation.
In the U.S. Pat. No. 3,944,910 of R. Rau, assigned to the same assignee as this application, there is disclosed an investigating apparatus capable of determining, inter alia, the dielectric constant of formations surrounding a borehole by injecting microwave electromagnetic energy into the formations and measuring the relative phase shift and attenuation of the wave energy as it propagates through the formations. This apparatus has demonstrated its effectiveness as a well logging tool, but certain practical limitations pertaining to frequency of operation, antenna spacing, etc., result in that well logging tool having its main application in determining the dielectric constant of formations relatively near the surface of the borehole wall. Stated another way, the microwave electromagnetic propagation device described in U.S. Pat. No. 3,944,910 is a relatively "shallow" investigation tool which primarily determines characteristics of the "invaded zone" surrounding the borehole, this being the zone in which borehole drilling fluids typically have displaced at least a portion of the fluids originally present in the formations. The microwave frequencies employed in the Rau patent render it difficult to investigate deeper than the invaded zone since the relatively longer transmitter to receiver spacings needed for a deeper investigation tend to become impractical since microwave signals attenuate relatively quickly in the formations. While information concerning the invaded zone can be extremely valuable, it would be additionally advantageous to obtain an indication of the dielectric constant of formations which are further from the borehole; i.e. in the virgin or "uninvaded" formations, or at least formations which are subject to less invasion than the formations in very close proximity to the borehole.
Even before development of the techniques disclosed in the referenced U.S. Pat. No. 3,944,910, it had been proposed that propagating electromagnetic energy, at frequencies typically below the microwave range, could be injected into the formations with a view toward measuring the properties of propagation of the energy in the formations. For example, in the U.S. Pat. No. 3,551,797 of Gouilloud et al., there is disclosed a technique wherein electromagnetic energy is transmitted into the formations and energy shed back into the borehole is measured at two spaced receivers to determine the relative attenuation and/or the relative phase of the electromagnetic energy propagating in the formations. Gouilloud et al. teaches that by using different transmitter-to-receiver spacings, different depths of investigation into the borehole can be attained. For example, a relatively closer spaced receiver pair can be utilized to obtain attenuation and/or phase information from which properties of the invaded zone are determined and measurements of attenuation and/or phase from a relatively further spaced pair of receivers can be utilized to obtain the properties of the deeper uninvaded formations. In the patent of Gouilloud et al., the concern is largely with obtaining conductivity. Either attenuation or phase can be utilized therein to determine the skin depth of the formations, with conductivity then being determinable from the skin depth. Below a certain frequency range, the skin depth of the electromagnetic energy can be calculated using either attenuation or phase information since displacement currents have minimal effect.
Reviewing up to this point, the prior art discussed so far shows that electromagnetic energy propagated in formations of interest can be measured to determine the conductivity of the formations (e.g. the Gouilloud et al. U.S. Pat. No. 3,551,797), and much higher microwave frequency electromagnetic energy can be propagated in the formations, and especially the invaded zone thereof, to determine the dielectric constant thereof (e.g. the Rau U.S. Pat. No. 3,944,910). There have also been various proposals for using electromagnetic energy at frequencies intermediate those discussed so far, i.e. radio frequency electromagnetic energy in the range between about 10 MHz and 100 MHz, to determine the dielectric constant and/or the conductivity of formations surrounding a borehole. In this frequency range, dielectric constant and conductivity both have a substantial effect upon the propagation constant of electromagnetic energy propagating in the formations, so measurements of attenuation and phase can be used for solution of simultaneous equations to determine the dielectric constant and/or conductivity of formations through which the electromagnetic energy has passed. Also, in this frequency range signal attenuation is much less severe than in the case of microwave electromagnetic energy, so transmitter-to-receiver spacings can be substantially greater with concomitant improvements in depth of investigation. The use of frequencies in the radio frequency range above 10 MHz is disclosed, for example in various Russian publications: e.g. Daev "Dielectric Induction Logging" Izv. MVO SSSR, Ser. Geologiya Razvedka (1965); Antonov and Daev "Equipment for Dielectric Induction Logging", Geofiz. Apparatura, No. 26 (1965); Antonov and Izyumov "Two Frequency Dielectric Induction Logging with Two Sondes", Geol. Geofiz., No. 4 (1968); Daev "Physical Principles of Electromagnetic Wave Logging", Geol. Razved, No. 4 (1970). More recently, a number of patents have issued, among them U.S. Pat. Nos. 3,891,916; 3,982,176; 3,893,021; 3,982,176; 3,993,944; 4,009,434 and 4,012,689, which utilize electromagnetic energy in the radio frequency range between about 10 MHz and 60 MHz to determine the dielectric constant and/or the conductivity of formations surrounding a borehole. Briefly, the techniques in the Russian publications and the listed patents generally recognize that dielectric constant and conductivity are two unknowns in the wave propagation equation. A basic approach is to establish two or more equations from which the unknowns can be simultaneously solved. In one instance, the amplitude and phase of wave energy are each measured so that two equations can be set up. In another instance, the amplitude at two different spacings is utilized, in a further instance conductivity is obtained from a low frequency induction device and measurements in the radio frequency range are utilized as other inputs in solving for dielectric constant.
In the described prior art techniques, the depth of investigation of a particular logging device, at a given frequency, is generally understood to be determined by transmitter-to-receiver spacings. At least two types of basic considerations pertaining to depth of investigation are evident in the prior art. First, when it is desired to obtain values of formation characteristics such as conductivity or dielectric constant, different spacings can be intentionally used to determine these characteristics at different depths of investigation (as discussed briefly above). For example, a relatively short spacing logging device can be used to measure formation characteristics in the invaded zone and a relatively long spacing logging device can be used to measure formation characteristics in the non-invaded zone. Secondly, in some techniques measured values are, of necessity, indicative of readings at different depths of investigation, such as where different logging devices are employed to obtain different formation characteristics that are utilized together in formation evaluation. When the measured values are combined to yield formation characteristics, the different depths of investigation are considered as introducing error. This would appear to follow logically since some of the readings used to evaluate formations may be coming from one depth of investigation and other readings coming from another depth of investigation where the formations might be of a different nature.
It is an object of the present invention to provide an apparatus and method for investigating earth formations utilizing radio frequency electromagnetic energy, the apparatus and method employing an improved technique for determining parameters such as dielectric constant and/or conductivity with assurance that the measurements used for such determination are "looking" at substantially the same formations; i.e., having substantially the same depth of investigation.